The traditional a‐factor model has focused on the occurrence frequencies of common cause failure (CCF) events. Global a‐factors in the a‐factor model are defined as fractions of failure probability for particular groups of components. However, there are unknown uncertainties in the CCF parameters estimation for the scarcity of available failure data. Joint distributions of CCF parameters are actually determined by a set of possible causes, which are characterized by CCF‐triggering abilities and occurrence frequencies. In the present paper, the process of a‐decomposition (Kelly‐CCF method) is developed to learn about sources of uncertainty in CCF parameter estimation. Moreover, it aims to evaluate CCF risk significances of different causes, which are named as decomposed a‐factors. Firstly, a Hybrid Bayesian Network is adopted to reveal the relationship between potential causes and failures. Secondly, because all potential causes have different occurrence frequencies and abilities to trigger dependent failures or independent failures, a regression model is provided and proved by conditional probability. Global a‐factors are expressed by explanatory variables (causes’ occurrence frequencies) and parameters (decomposed a‐factors). At last, an example is provided to illustrate the process of hierarchical Bayesian inference for the a‐decomposition process. This study shows that the a‐decomposition method can integrate failure information from cause, component and system level. It can parameterize the CCF risk significance of possible causes and can update probability distributions of global a‐factors. Besides, it can provide a reliable way to evaluate uncertainty sources and reduce the uncertainty in probabilistic risk assessment. It is recommended to build databases including CCF parameters and corresponding causes’ occurrence frequency of each targeted system.
